1) Field of the Invention
This invention relates to a process for the production of a foamed polyolefin sheet.
2) Description of the Related Art
Conventionally, foamed polyolefin sheets have each been produced by extruding a polyolefin resin, which contains a blowing agent, into a low-pressure zone through an annular die provided at an end of an extruder to form a foamed tubular body, bringing the foamed tubular body into contact with a peripheral surface of a cylindrical cooling device to cool the foamed tubular body and then slitting the thus-cooled foamed tubular body in the direction of extrusion to open same. In this conventional process, it is however difficult to increase the ratio (blow ratio) of the diameter of the annular die to the diameter of the cylindrical cooling device because the extensibility of the extruded foamed tubular body drops abruptly. When the blow ratio is small, the foamed tubular body cannot be extended sufficiently. As a result, corrugations occur in the resulting foamed sheet, resulting in problems such that these corrugations deteriorate the external appearance of the product, lower the accuracy upon cutting the sheet and also reduce the accuracy of bonding namely, form portions having good strength and those having poor strength when employed for lamination. It is preferred that a blowing agent employed for the production of such foamed sheets is inexpensive. Economical butane is suitable from this standpoint. The use of a blowing agent having a high foaming speed, like butane, is accompanied by the problem that the occurrence of corrugations becomes particularly remarkable. It has therefore been desired to develop a process which can provide foamed polyolefin sheets, which contain less corrugations, by using an inexpensive blowing agent such as butane.